The invention relates generally to adaptive "single-bit" (delta and delta-sigma modulation) digital encoding and decoding systems for high-quality audio signals in which the message frequency band extends from very low audio frequencies, in the order of 20 to 50 Hz, to about 15 kHz. However, the invention is not limited to such applications. In particular, the invention relates to such systems in which the adaptive function, by means of an adaptive filter, variably divides the message frequency band into delta and delta-sigma modulation regimes of operation. The invention also relates to a circuit for providing an adaptation control signal responsive to the information in the bit stream ("bit-stream loading").
A simple single-integration delta modulator is a type of single-bit digital encoder that encodes an audio signal as a series of ones and zeros in such a way that the average number of ones over a short period of time represents the instaneous slope of the audio signal. Each one-bit word of the bit stream tells the delta modulation decoder to take one step up or down to reconstruct the audio. The size of this step is a parameter of the design: small steps give small quantization errors but limit the maximum slope of the signal, and steps large enough to accommodate high-frequency high-level signals produce large quantization errors. An adaptive delta modulator changes the step size dynamically, attempting to provide an acceptable compromise between quantization-error level and high-frequency signal handling ability.
In a delta-sigma modulator, which is also a single-bit digital device, the average number of ones over a short period of time represents the audio signal itself instead of its slope. Thus, the delta-sigma modulator, unlike the delta modulator, has an overload characteristic that is independent of frequency. Adaptive delta-sigma modulation systems are also well known.
A common way to implement delta modulation encoders and decoders is to obtain the integration function by using a fixed-frequency low-pass filter having a corner (pole) frequency low in the message signal band (300 Hz in a prior art high-quality audio delta-modulation system). Such an arrangement is sometimes referred to as a "leaky integrator": below the corner (turnover) frequency of the filter (the "leak" frequency or "leak time constant"), the modulator acts as a delta-sigma modulator and, above the corner frequency, the modulator acts as a delta modulator. In adaptive systems, the gain of the fixed-frequency low-pass filter integrator is varied to achieve the adaptation. It appears that circuit designers have used leaky integrators for two principal reasons: a leaky integrator, unlike a pure integrator, does not require infinite gain at low frequencies; and a leaky integrator, unlike a pure integrator, quickly dissipates errors in the bit stream due to its relatively short time constant.
The invention recognizes that the division of the message signal band into delta modulation and delta-sigma modulation regimes of operation is desirable, but that further improvement in performance and simplicity of operation can be achieved by dynamically varying the frequency at which the message band is divided into the two regimes of operation. This is accomplished by dynamically varying the corner (pole) frequency of the leaky integrator as the adaptive parameter of the system and by allowing, under certain signal conditions, the corner frequency of the leaky integrator to assume frequencies relatively high in the message signal band.
In any adaptive single-bit digital encoding and decoding system, circuitry is required to determine the amount of adaptation needed at any given time. Many prior art adaptive delta modulators operate in the digital domain to determine the required adaptation, using various bit-counting algorithms and circuits to increase the step size when long strings of ones or zeros are encountered in the encoded digital audio bit stream. Other prior art delta-modulator adaptation-control circuits operate in the analog domain, typically employing techniques similar to those used in the control circuits of analog audio compressors and expanders, including the use of "speed-up" networks to minimize overload at the onset of transients. The control circuit of the present invention, although operating also in the analog domain, recognizes that the encoded digital bit stream carries audio information particularly well suited for adaptive control and that the audio information can be simply derived and processed as an analog signal for use as an adaptation control signal.